1. Field of the Invention
This invention relates to an apparatus and process for the catalytic cracking of hydrocarbons. The apparatus comprises two independent transfer line reactors, each of which is associated with an independent cyclone separation system and wherein the cyclone separation systems are located within a common reactor vessel. More particularly, the invention relates to an improved apparatus which can be used for the simultaneous fluidized catalytic cracking of dissimilar hydrocarbon feedstocks without commingling either the feedstocks or the products therefrom.
2. Description of the Prior Art
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels which are utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
Products from the catalytic cracking of heavy petroleum fractions have been of suitable quality for direct use as gasoline blending components. However, the recent trend toward lower sulfur and lead levels in gasoline has made some form of reprocessing of the catalytic cracking product desirable. In addition, requirements for additional sources of light olefins for either chemical feedstock or alkylate feedstock and of light aromatics for chemical feedstock have also made reprocessing desirable.
U.S. Pat. No. 3,928,172 to Davis et al. discloses a method for improving the quality and volatility of a refinery gasoline pool which comprises catalytically cracking a gas oil, fractionating the resulting products, and recracking the heavy naphtha which is obtained from the fractionation. This patent discloses a catalytic cracking apparatus which comprises two independent vertical transfer line reactors wherein one transfer line reactor is for gas oil cracking and the other is for heavy naphtha recracking. Each transfer line reactor is directly connected to a separate cyclone separation system, and the hydrocarbon products flow from each cyclone system to a separate product recovery system to prevent the mixing of products from the two reactors. The spent catalyst from each cyclone separation system flows downwardly through dip-legs and into a bed of spent catalyst within a stripping vessel. The cyclone system for the gas oil reactor is located within the stripping vessel, whereas the cyclone system for the heavy naphtha reactor is located outside of the stripping vessel and spent catalyst from it flows downwardly through a dip-leg which terminates in the stripping vessel and is positioned at an angle of about 45.degree. from vertical.
The apparatus which is disclosed by the Davis et al. patent is unsatisfactory, however, in many respects. When a cyclone dip-leg is positioned at a substantial angle from vertical, as is required by the Davis et al. disclosure, the downward flowing catalyst causes a relatively rapid and highly undesirable erosion of the lower interior surface of the dip-leg which can ultimately result in the formation of a hole in the dip-leg wall. Such a hole would, of course, require that the process unit be shut down if it is formed at a location which permits hydrocarbons and catalyst to escape from the process unit. In addition, Davis et al. fail to disclose or suggest any means which would permit the products from both transfer line reactors to be commingled and sent to a single product recovery system in the event that this should become either necessary or desirable, for example, as a consequence of equipment failure or a need to process an identical feedstock in both transfer line reactors. Further, Davis et al. fail to teach or suggest either the possibility or desirability of placing both cyclone systems within a single main reactor vessel.
U.S. Pat. No. 4,067,798 to Hauschildt et al. also discloses a method for improving the quality of a refinery gasoline pool. Hauschildt et al. teach that a feedstock comprising from 80 to 100 volume percent of a hydrocarbon fraction containing at least 25 weight percent of mono-aromatic hydrocarbons and boiling in the boiling range of heavy gasoline in combination with from 0 to 20 volume percent of a vaporizable hydrocarbon fraction can be catalytically cracked to produce a product having an increased low molecular weight olefin and mono-aromatic content, a substantially reduced sulfur content, and a high octane rating relative to the feedstock. It is disclosed that the fraction comprising from 80 to 100 volume percent of the feedstock is preferably a catalytic cracking product. It is also disclosed that the feedstock must be subjected to catalytic cracking in the absence of any other feedstock and the products must be isolated separately from any other products. Although the Hauschildt et al. process can be carried out using conventional apparatus for the catalytic cracking of hydrocarbons, there is a need for an apparatus which will permit the more efficient practice of this process.